Technical Field
Embodiments of the invention relate generally to magnetic resonance imaging (MRI) and, more specifically, to a system and method for using matrix shim coils to generate magnetic fields with low to high order spatial harmonics.
Discussion of Art
MRI is a widely accepted and commercially available technique for obtaining digitized visual images representing the internal structure of objects having substantial populations of atomic nuclei that are susceptible to nuclear magnetic resonance (NMR). In MRI, imposing a strong main magnetic field (B0) on the nuclei polarizes nuclei in the object to be imaged. The nuclei are excited by a radio frequency (RF) signal at characteristics NMR (Larmor) frequencies. By spatially disturbing localized magnetic fields surrounding the object and analyzing the resulting RF responses from the nuclei as the excited protons relax back to their lower energy normal state, a map or image of these nuclei responses as a function of their spatial location is generated and displayed. An image of the nuclei responses provides a non-invasive view of an object's internal structure. Additionally, Many MRI systems use electromagnetic gradient coils to produce small amplitude, spatially varying magnetic fields. Typically, the magnetic component produced by gradient coils is aligned along the z-axis of the MRI system and varies linearly in amplitude with position along one of the x, y or z axes of the MRI system. Accordingly, many MRI systems use gradient coil to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis.
In a typical MRI system, the more uniform the B0 field, the better the quality of images produced. Inhomogeneities may be introduced into the B0 field (B0 field inhomogeneities) by various factors, however, such as manufacturing tolerances, installation errors, environmental effects, design restrictions, imperfections in one or more magnet(s), ferromagnetic material near the installation site, and/or other sources of electromagnetic noise/interference. For example, time varying magnetic fields can produce B0 field inhomogeneities by generating currents, known as eddy currents, within the MRI machinery and/or the subject being imaged. In turn, the generated eddy currents may produce additional magnetic fields, known as eddy current fields and/or reflective fields, which can distort the B0 field and degrade image quality.
In order to compensate for image degradation caused by eddy currents, and/or other sources of B0 field inhomogeneities, many MRI systems implement a technique known as “pre-emphasis” to reduce the effect of B0 field inhomogeneities. Traditional MRI systems achieve pre-emphasis by modulating the current in the gradient coils in an attempt to mitigate the distortion of the B0 field due to B0 field inhomogeneities. The geometric shapes which gradient coils can modulate is limited, which in turn limits the spatial harmonics order of B0 field inhomogeneities that can be compensated for.
What is needed, therefore, is a system and method that improves overall imaging performance and, in particular, provides for the mitigation of high order spatial harmonic B0 field inhomogeneities.